The dock area at Calais is not dark and unprotected - it's just like a campsite, or parking area - with rows of caravans and motorhomes waiting for ferries in, or ferries out, and absolutely safe - with showers and toilets, patrols and security. However, just like Jazman we've been sleeping on French aires for 25 years with no problem - and ALWAYS catch a late-night ferry and put our heads down on an aire somewhere about an hour or couple of hours from the port. No doubt people are robbed, sometimes, but there are spurious claims (and some of them have been originated by the 'gas alarm' industry trying to find a market for it's products). There are probably some insurance scams too - we're convinced our neighbours were involved in one, once, when they were reported by a caravan magazine to have been 'gassed' just outside Bordeaux. They denied that it had happened when they returned home!
Like previously said, there are so many places in a van to tuck away your valuables, and no sneak thief is going to find our money, passports, or anything else of value. One tip read somewhere else was to leave a wallet with a few euro coins, and a stack of 'expired' credit cards or business cards prominently somewhere - and hide the real thing well away from prying hands!